Philadelphia: Temperature soared to more than 38 degrees Celsius in several cities as Americans dipped into the water, went to the movies and rode the subway just to be in air conditioning for relief from unrelenting heat that has killed at least 30 people across half the country.

The heat on Saturday sent temperature above 38 degrees Celsius in cities including a record 40.5 C in Washington, St Louis (41 C), and Indianapolis (40 C), buckled highways and derailed a Washington-area train even as another round of summer storms threatened.

Temperature soared in more than 20 states to 40.5 C in Louisville, Kentucky, 38.5 C in Philadelphia, and 35 C in New York; besides Washington, a record of 40 C was set in Sioux Falls, South Dakota, and Baltimore set a record at 39 C.

At least 30 deaths were blamed on the heat, including nine in Maryland and 10 in Chicago, mostly among the elderly. Three elderly people found dead in their houses in Ohio had heart disease, but died of high temperature in homes lacking power because of recent outages, officials said. Heat was also cited as a factor in three deaths in Wisconsin, two in Tennessee and three in Pennsylvania.

Officials said the heat caused highways to buckle in Illinois and Wisconsin. In Maryland, investigators said heat likely caused rails to kink and led a green line train to partially derail in Prince George's County, Maryland, on Friday afternoon. No one was injured, and 55 passengers were safely evacuated.

Thousands of mid-Atlantic residents remained without power more than a week after deadly summer storms and extreme heat struck the area, including 120,000 in West Virginia and some 8,000 in the suburbs around Baltimore and Washington, DC.

In the Washington area, the utility company Pepco asked customers to conserve power, saying the heat was stressing the system. "This is becoming a black swan of heat waves, in the sense that it's such a long heat wave, such a severe heat wave and encompassing such a large area," said Chris Vaccaro, spokesman for the National Oceanic and Atmospheric Administration. (Agencies)